Add HTTP (streamable) transport to MCP client + inline MCP tool rendering

[stippi]

What

Adds an HTTP (streamable) transport to MCP client mode, alongside the existing stdio transport, and reworks how MCP tool blocks render in the gpui UI.

MCP client — HTTP transport

• McpServerConfig now carries a typed, #[serde(untagged)] McpTransport enum. The JSON shape selects the transport:
  • an object with a url (plus optional headers) → HTTP
  • an object with a command (plus args/env) → stdio
  • Existing mcp-servers.json files stay valid (backward compatible).
• ${VAR} substitution now applies to HTTP headers values as well as stdio env values.
• The client builds a reqwest-backed StreamableHttpClientTransport with custom headers (rmcp feature transport-streamable-http-client-reqwest + reqwest).
• connect_stdio() / connect_http() are symmetric private helpers behind connect().
• The gpui MCP Servers settings section gained a transport selector (command/args/env vs url/headers).

Example config:

{
  "servers": {
    "jira":   { "command": "npx", "args": ["-y", "some-jira-server"] },
    "remote": { "url": "https://example.com/mcp",
                "headers": { "Authorization": "Bearer ${REMOTE_TOKEN}" } }
  }
}

gpui — MCP tool block rendering

Previously MCP tools (dynamic mcp__<server>__<tool> names) had no registered renderer and fell back to a raw [name] placeholder. Now:

• New McpToolRenderer (inline/lightweight style), installed as the registry's MCP fallback via ToolBlockRendererRegistry::resolve.
• Generic MCP icon + bare tool name (get_me); the server name (github_tools_sap) shows as a right-aligned pill via a new ToolBlockRenderer::header_tag() hook.
• Tool JSON output is pretty-printed and rendered in a monospace font; non-JSON output is shown verbatim.

Tests

• Config: round-trip + ${VAR} substitution for both transports.
• New in-process axum HTTP server integration test (connects_and_calls_over_http) exercising the full HTTP path: connect, tools/list, tools/call.
• pretty_json / parse_mcp_name unit tests.
• cargo fmt, clippy -D warnings, and the touched crates' tests are green; validated end-to-end against a real HTTP MCP server.

Note

This branch also contains the pre-existing, unrelated commit 26875a47 ("Fix previous_response_not_found after WebSocket reconnect"), which was already on the branch before this work.